Objects having perforated patterns are used in many applications for providing various functions. Some objects include perforated patterns that facilitate relatively non-impeded flow through the objects. For example, air vent covers having perforated patterns are used in computing devices to facilitate the passage of air flow into and out of the computing devices, such as for cooling processing components of the devices. Air vent covers are also used to attenuate predefined acoustic noise and electromagnetic interference generated by computing devices. In other applications, perforated patterns are used directly as filters for filtering objects having a particular size from other objects having a larger size, or as support or backing for finer particulate filters to prevent bowing of the filter media as objects accumulate on the finer particulate filters.
Perforated patterns in objects, such as air vent covers and filters, have been designed to achieve various particular attributes. Often, however, one or more of the particular attributes are achieved at the expense of one or more other attributes.
Generally, the larger the free-area coefficient, i.e., the ratio of open area to total area, the lower the flow impedance caused by the vent at a given flow rate. Moreover, the lower the impedance, the lower the pumping power required to move a fixed amount of air through the perforations, which in the context of air vent covers in computing devices, typically results in a reduced amount of electrical power for operating one or more cooling fans of the computing device and lower fan induced acoustic noise. For example, typical net efficiencies of computer air moving devices, including motor, mechanical, and aerodynamic losses, require the net expenditure of electrical energy at a rate that is on the order of 2.5 to 5 times that of the original fluid pumping power.
In practice, the free-area coefficient of perforated patterns is limited by several factors. For example, in computing device applications having components with increasing clock speeds, to comply with electromagnetic compatibility (EMC) or electromagnetic interference (EMI) reduction requirements, the higher frequencies and corresponding shorter wavelengths associated with faster clock speeds require the maximum aperture size of the perforations be made smaller. Additionally, fabrication requirements limit the web thickness achievable for a given material thickness. The changing EMC reduction requirements and minimum allowable web thickness limitations effectively reduce the free-area coefficient of conventional perforated patterns, which increases the impedance of the perforated patterns at a given volumetric flow rate.
Depending on the application in which perforated patterns are used, the strength and stiffness of the object in which perforated patterns are formed can be important. For example, air vent covers may add structure and rigidity to a particular object, such as the door of a rack for storing computing devices. Additionally, filters must be sufficiently strong to withstand various impact forces caused by filtered and unfiltered objects. Therefore, in certain applications, perforated patterns should be designed to provide structure and rigidity, as well as to withstand impact forces if applicable.
Typical perforated patterns used in air flow and filtering objects include hexagonal, circular, and square, with each having certain advantages over the other. FIG. 1 shows a conventional perforated pattern 100 having circular shaped perforations 110 in a uniformly staggered arrangement formed in an object 120 and FIG. 2 shows a conventional perforated pattern 200 having hexagonal shaped perforations 210 also in a uniformly staggered arrangement formed in an object 220. Generally, perforation size and pitch (i.e., the number of perforations within a given area) are determined by employing simple equations based on aperture size and web thickness constraints to define a desirable opening to web area ratio for each pattern. Applying the equations under the same constraints, hexagonal perforated patterns, i.e., a repeating pattern of hexagonally shaped apertures, achieve the highest opening to web area ratio. Therefore, many air flow object manufacturers, such as computing device rack manufacturers, use vent covers that have conventional hexagonal perforated patterns.
Although hexagonal perforated patterns provide various advantages over other types of perforated patterns, e.g., circular and square perforated patterns, the other types of perforated patterns likewise can have certain advantages over hexagonal perforated patterns. For example, a hexagonal perforated pattern may have a higher opening to web area ratio, thus allowing a greater amount of air to flow through a vent cover, but a circular perforated pattern has higher shear and tensile stiffness. Therefore, it would be advantageous to provide a perforated pattern that achieves a desirable balance between these and/or other perforated pattern characteristics.